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Food Testing Guide for Students

This document provides instructions for performing common food tests to identify glucose, starch, protein and lipids. Samples must first be prepared by breaking up solids and mixing with water. Glucose is tested for using Benedict's solution, which turns from blue to orange/brick red with a positive result. Starch is tested for with iodine solution, which turns from orange-brown to blue-black in the presence of starch. Protein is tested for with Biuret solution, which turns from blue to violet/purple for a positive result. Lipids are tested for by mixing the sample with ethanol and adding water, which will cause a cloudy emulsion to form for a positive result. Safety precautions must be followed when

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Vandan Nagda
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142 views6 pages

Food Testing Guide for Students

This document provides instructions for performing common food tests to identify glucose, starch, protein and lipids. Samples must first be prepared by breaking up solids and mixing with water. Glucose is tested for using Benedict's solution, which turns from blue to orange/brick red with a positive result. Starch is tested for with iodine solution, which turns from orange-brown to blue-black in the presence of starch. Protein is tested for with Biuret solution, which turns from blue to violet/purple for a positive result. Lipids are tested for by mixing the sample with ethanol and adding water, which will cause a cloudy emulsion to form for a positive result. Safety precautions must be followed when

Uploaded by

Vandan Nagda
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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Practical: Food Tests

Preparing a sample

 Before you can carry out any of the food tests described below, you may need to
prepare a food sample first (especially for solid foods to be tested)
 To do this:
o Break up the food using a pestle and mortar
o Transfer to a test tube and add distilled water
o Mix the food with the water by stirring with a glass rod
o Filter the mixture using a funnel and filter paper, collecting the solution
o Proceed with the food tests

Test for glucose (a reducing sugar)

 Add Benedict's solution to the sample solution in a test tube


 Heat in a boiling water bath for 5 minutes
 Take the test tube out of the water bath and observe the colour
 A positive test will show a colour change from blue to orange / brick red

The Benedict's test for glucose


Test for starch using iodine

 We can use iodine to test for the presence or absence of starch in a food sample
 Add drops of iodine solution to the food sample
 A positive test will show a colour change from orange-brown to blue-black

In the presence of starch, iodine will turn from brown to blue-black


Test for protein

 Add drops of Biuret solution to the food sample


 A positive test will show a colour change from blue to violet / purple

The Biuret test for protein

Test for lipids

 Mix the food sample with 4cm3 of ethanol and shake


 Allow time for the sample to dissolve in the ethanol
 Strain the ethanol solution into another test tube
 Add the ethanol solution to an equal volume of cold distilled water (4cm3)
 A positive test will show a cloudy emulsion forming
The ethanol test for lipids

Food Test Results Table

Important hazards
 Whilst carrying out this practical you should try to identify the main hazards and
be thinking of ways to reduce harm
 Biuret solution contains copper (II) sulfate which is dangerous particularly if it
gets in the eyes, so always wear goggles
 Iodine is also an irritant to the eyes
 Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your
skin wash your hands immediately
 Ethanol is highly flammable; keep it away from any Bunsen burner
 The Bunsen burner itself is a hazard due to the open flame

Worked Example
Food tests: analysis

Write a conclusion to state which food groups are present one of the food samples you
tested and an explanation of how you know this.

Conclusion:

The apple contained both starch and sugar as it tested positive for both the iodine test
(orange → blue - black) and the benedict's test (blue → orange).

The apple did not contain protein or lipid (fat) as the biuret and emulsion tests were
both negative.

Applying CORMS to practical work


 When working with practical investigations, remember to consider your CORMS
evaluation.

CORMS evaluation

 In this investigation, your evaluation should look something like this:


o C - We are changing the type of food in the sample
o O - This is not relevant to this investigation as we aren't using an organism
o R - We will repeat the investigation several times for each food sample to
ensure a reliable result
o M1 - The presence of the specific biological molecule in each food type by
noting the colour change
o M2 - ....after testing with each specific testing agent
o S - We will control the volume of each testing agent used, the quantity of
the food sample, the concentration of the testing agents, the temperature
of the water bath for the Benedicts test. There may be other examples that
you can think of

Exam Tip
When describing food tests in exam answers, make sure you give the starting colour of
the solution and the colour it changes to for a positive result.

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